Current confinement structure for vertical cavity surface emitting laser

ABSTRACT

A vertical cavity surface emitting laser (VCSEL) structure and fabrication method therefor are described in which a subsurface air, gas, or vacuum current confinement method is used to restrict the area of electrical flow in the active region. Using vertical hollow shafts to access a subsurface current confinement layer, a selective lateral etching process is used to form a plurality of subsurface cavities in the current confinement layer, the lateral etching process continuing until the subsurface cavities laterally merge to form a single subsurface circumferential cavity that surrounds a desired current confinement zone. Because the subsurface circumferential cavity is filled with air, gas, or vacuum, the stresses associated with oxidation-based current confinement methods are avoided. Additionally, because the confinement is achieved by subsurface cavity structures, overall mechanical strength of the current-confining region is maintained.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Provisional Application Ser. No.60/278,715, filed Mar. 26, 2001, which is incorporated by referenceherein.

FIELD

This patent specification relates to semiconductor lasers. Moreparticularly, it relates to vertical cavity surface emitting lasers(VCSELs) in which electric current requires lateral confinement inpassing through an active region.

BACKGROUND

Solid-state semiconductor lasers represent desirable light sources for avariety of applications including optical data communications,telecommunications, and other applications. A vertical cavity surfaceemitting laser (VCSEL) is a solid-state semiconductor laser in whichlight is emitted from the surface of a monolithic structure ofsemiconductor layers, in a direction normal to the surface. This is incontrast to the more commonly used edge-emitting laser, in which lightis emitted from the edge of the wafer. Whereas edge-emitting lasers relyon facet mirrors formed at the wafer edge by cleaving or dry etching,the operation of VCSELs is enabled through the use of distributed Braggreflector (DBR) mirrors for longitudinal optical confinement. VCSELs areadvantageous over edge-emitting lasers in that (i) they have alower-divergence, circularly-shaped laser beam, (ii) may be manufacturedusing standard fabrication processes such as those used in silicon VLSItechnology, (iii) may be tested at the wafer level prior to packaging,and (iv) may be fabricated in dense 2-dimensional arrays for lower costand higher volume.

As in any laser, the overall structure of a VCSEL is that of two endmirrors on each side of an active region, the active region producingthe light responsive to an electric current therethrough. However, theactive region is a thin semiconductor structure, and the end mirrors aredistributed Bragg reflector mirrors (“DBR mirrors”) comprisingalternating layers of differently-indexed material such that light ofonly the desired wavelengths is reflected. Further general informationon VCSELs may be found in the following references, each of which isincorporated by reference herein: Cheng and Dutta, eds., Vertical-CavitySurface-Emitting Lasers: Technology and Applications, Vol. 10 ofOptoelectronic Properties of Semiconductors and Superlattices, Manasreh,ed., Gordon and Breach Science Publishers (2000); Sale, T. E., VerticalCavity Surface Emitting Lasers, Wiley & Sons (1995); and Dutton,Understanding Optical Communications (Prentice Hall 1998), at pp.159-161.

Today, VCSELs are widely used in local area networks and in veryshort-reach applications such as connections between electronic routers,the back planes of computers, computers to disk farms and computers tosensors, and curb to home, at wavelengths of about 650 nm to 980 nm. Forlong-distance fiber optic communications applications, e.g. generallyrequiring transmission distances of greater than 10 km, VCSELS havinglonger output wavelengths of 1300-1550 nm will be useful. Today,commercially-available VCSELs are directly modulated to frequencies of2.5 Gb/s, while direct modulation of up to 20 Gb/s has been reached inresearch laboratories. With increasing demands for bandwidth, it becomesdesirable to develop practical and reliable VCSELs with modulationfrequencies of greater than 10 Gb/s.

One problem in the practical realization of higher VCSEL modulationfrequencies exists in regard to current confinement, which relatesgenerally to the confinement of electrical current passing through theactive layer to a small lateral portion of that layer to increasecurrent density. At higher modulation frequencies, it is necessary toincrease the photon density in the optical cavity to increase therelaxation oscillation frequency of the laser. The increase in photondensity necessitates an increase in the injected current density of thelaser which, in turn, increases heating at the active layer. Dependingon the laser structure, this increase in heat can cause the laser tohave a reduced lifetime.

Proper current confinement of the injected electrons and holes to theactive area is important to achieve a highly efficient laser. It is notuncommon for a VCSEL to have a desired “wall plug” efficiency, i.e., theratio of optical power emitted by the laser over the electrical powerapplied, in excess of 50%. High wall plug efficiency is necessary forhigh frequency operation to minimize excess heating. Several methods arein use to achieve proper current confinement, including protonimplantation methods, mesa methods, buried regrowth methods, andoxide-based methods. Disadvantages exist with these methods, however.For example, one problem with proton implantation is that it cannotdefine a small sharp region due to the nature of ion implantation. Asanother example, in the mesa method, the mesa is non-planar and hasdifficulty in passivation, resulting in poor reliability. As anotherexample, buried regrowth structures are more complex to manufacture andresult in a reduced yield.

Oxide confinement, in which layers adjacent to the active layer areselectively oxidized such that current is confined to a smallnon-oxidized portion, is generally the most widely used method tomanufacture low threshold and efficient VCSELs. See Jewell et. al.,“Vertical cavity surface emitting lasers: design, growth, fabrication,characterization”, IEEE Journal of Quantum Electronics, vol. 27, no. 6,pp. 1332-1346 (June 1991), which is incorporated by reference herein.Oxide-confined VCSELs have achieved sub-milliampere threshold currentand single mode operation. See, e.g., Deppe et al, “Low-thresholdvertical cavity surface emitting lasers based on oxide confinement andhigh contrast distributed Bragg reflectors,” IEEE Journal of SelectedTopics in Quantum Electronics, vol. 3, no. 3, pp. 893-904 (June 1997);Nishiyama et. al., “Multi-oxide layer structure for single modeoperation in vertical cavity surface emitting lasers,” IEEE PhotonicsTechnology Letters, vol. 12, no. 6, pp. 606-8 (June 2000); and Deppe,“Optoelectronic Properties of Semiconductors and Superlattices,” atChapter 1 of Cheng and Dutta, supra, each of these references beinghereby incorporated by reference herein.

Oxide confinement methods have some characteristics that may result inreduced lifetime of the laser at high current injection operations, asis needed for high modulation frequencies. For example, according to oneoxide confinement method discussed in Choquette, “The Technology ofSelectively Oxidized Vertical Cavity Lasers,” at Chapter 2 of Cheng andDutta, supra, which is incorporated by reference herein, the highcontent of Al in an AlGaAs or AlAs layer is oxidized using nitrogen andsteam at a temperature of 420-450 degrees Celsius. However, the AlGaAsor AlAs layer undergoes a dimensional change after the oxidation. Thisdimensional change causes stresses in the current confinement area, aswell as in the active area, which is in close proximity to the currentconfinement area. These stresses, as well as the method of fabricatingthe oxide confining layer for the VCSEL, are discussed in Choquette,supra.

Accordingly, it would be desirable to provide a VCSEL having a currentconfinement structure with advantages similar to those of the oxideconfinement method, while at the same time avoiding the disadvantages ofthe material stresses caused by the oxide confinement method that mayreduce the lifetime of the VCSEL device.

SUMMARY

According to a preferred embodiment, a VCSEL structure and fabricationmethod therefor are provided, wherein a subsurface air, gas, or vacuumcurrent confinement method is used to restrict the area of electricalflow in the active region. Because air, gas, or vacuum is used, thestresses caused in oxidation-based current confinement methods areavoided. Additionally, because the confinement is achieved by subsurfacecavity structures, overall mechanical strength of the current-confiningregion is maintained, such that possible “collapse” of thecurrent-confining structure is not a problem.

According to a preferred embodiment, a vertical cavity structure isformed comprising a lower distributed Bragg reflector (DBR), an upperDBR, an active layer positioned between the lower DBR and the upper DBR,and a current confinement layer positioned adjacent to the active layer.The current confinement layer comprises a semiconductor material that islaterally etchable by a selective etchant to which the upper DBRmaterial and active layer materials are etch-resistant. Three or morehollow vertical shafts are formed through the upper DBR layer andcurrent confinement layer, the hollow vertical shafts being positionedoutside a desired current confinement zone, the centers of the hollowvertical shafts forming a polygon that laterally circumscribes an areaaround the desired current confinement zone. The selective etchant isthen applied, laterally etching away the current confinement layeroutwardly from the axis of each hollow vertical shaft, forming asubsurface cavity around each hollow vertical shaft at the currentconfinement layer. The lateral etching process is continued until thesubsurface cavities merge together to form a single subsurfacecircumferential cavity around the desired current confinement zone.Electrical current is confined to the current confinement zone as itpasses through the current confinement layer. After optionally fillingthe subsurface circumferential cavity with an inert gas or other inertnon-solid material, the hollow vertical shafts are sealed off by apolyimide material, a gold plating material, and/or a dielectricmaterial.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a side view of a vertical cavity surface emittinglaser (VCSEL) according to a preferred embodiment;

FIG. 2 illustrates an exploded perspective view of a current confinementstructure according to a preferred embodiment;

FIG. 3 illustrates an exploded perspective view of a current confinementstructure according to a preferred embodiment; and

FIGS. 4-8 illustrate the VCSEL of FIG. 1 during different stages of thedevice fabrication process.

DESCRIPTION

According to one preferred embodiment, to overcome the limitations ofthe oxide-confinement method and the mesa-confinement method, a subsurface air aperture method is provided. Instead of oxidizing the layernear the active region (e.g., the AlGaAs layer) as in hole-definedoxidation (see Chua, C. L. et. al., “Planar laterally oxidizedvertical-cavity lasers for low-threshold high-densitytop-surface-emitting arrays,” IEEE Photonics Technology Letters, Vol. 9,No. 8, pp. 1060-2 (August 1997), which is incorporated by referenceherein; see also Choquette, supra at p. 86), that layer is selectivelyetched away in a sub-surface etching process until only a verticalcolumn, laterally circumscribed and isolated by an air cavity, remainsat that layer beneath the surface of the wafer. Electrical current istherefore confined to the vertical column when passing from above thatlayer to below that layer.

The method of the preferred embodiments is in contrast to simply etchinga mesa, as in earlier VCSEL fabrications (see Jewell, supra at p. 1338),where the process is highly nonplanar, resulting in manufacturingdifficulty, and where the exposed mesa is difficult to passivate,resulting in degradation of the mesa due to oxidation of the AlGaAslayers and electrical degradations such as increases in leakage current.In a conventional mesa structure, due to non-uniform etching rates ofthe various layers, cantilever structures may result. Such cantileveredstructures can collapse, resulting in a structure that is not desirablein the manufacturing of a reliable, reproducible VCSEL. See Deppe, supraat p. 89, where such a collapsed structure is shown.

FIG. 1 illustrates a side view of a vertical cavity surface emittinglaser (VCSEL) 100 according to a preferred embodiment. VCSEL 100comprises a substrate 102, a lower DBR 104, an n-doped semiconductorlayer 106, an active layer 108 such as a multi-quantum well (MQW)structure, a current confinement layer 112, a p-doped semiconductorlayer 114, an upper DBR 116, an upper ohmic contact 120, a lower ohmiccontact 122, and an antireflection-coated aperture 124. In the currentconfinement layer 112 is formed a subsurface circumferential cavity 128that surrounds a current confinement zone 126. Current is confined tocurrent confinement zone 126 as indicated by conceptual current arrowsin FIG. 1. Outside the current confinement zone 126 an isolation region130 is formed by proton implantation or ion implantation that preventscurrent from flowing around the outside of the subsurfacecircumferential cavity 128. Vertical shafts 125 used during theformation of the subsurface circumferential cavity 128 are plugged withinert spacer material such as a dielectric material 118. As used herein,the term shaft is used to denote a hole (like a mine shaft) rather thanto denote a solid member (like a drive shaft). Other materials may beused to seal the vertical shafts 125 such as polyamide material.Alternatively, the upper ohmic contact 120 may protrude somewhat intothe vertical shafts 125 to seal them and therefore provide the sealingfunction. The subsurface circumferential cavity 128 is preferably sealedto contain an inert gas such as N₂, Ar, or He, but may alternativelycontain regular air or other nonsolid material.

FIG. 2 illustrates an exploded perspective view of the p-layer 114, thecurrent confinement layer 112, and the active layer 108. In theembodiment shown there are four vertical shafts 125 spaced at thevertexes of a 4-sided polygon, the vertical shafts 125 laterallycircumscribing the current confinement zone 126. In general, there maybe any number M>2 of vertical shafts 125 placed at roughly regularangular spacings of 360/M degrees around a center of the currentconfinement zone 126, the centers of the vertical shafts being locatedat the vertices of an M-sided polygon. However, in order to ensuremechanical stability of the device, there should not be so many verticalshafts 125 so as to cause them to merge together into a single “canyon.”

If the current confinement layer 112 is viewed in isolation, the currentconfinement zone 126 resembles a small “island” of semiconductormaterial surrounded by empty space that is the subsurfacecircumferential cavity 128. In the embodiment of FIG. 2, it is presumedthat the lateral etching of the current confinement layer 112 isanisotropic (directional) in two perpendicular lattice directions. Thecurrent confinement zone 126 is a column of semiconductor material,often of a somewhat irregular lateral shape, that extends from theactive layer 110 beneath the current confinement layer 112 to thep-doped semiconductor layer 116 above the current confinement layer 112.The section of current confinement layer 112 lying outside thesubsurface circumferential cavity 128, herein termed an outer supportelement 113, provides a mechanical support for the upper layers to keepthe shape and size of the subsurface circumferential cavity 128 intact.The outer support element 113 is ion-implanted, proton-implanted, orotherwise treated to be non-conducting, such that the current isrestricted to the current confinement zone 126.

FIG. 3 illustrates an exploded perspective view of a p-layer 114′, acurrent confinement layer 112′, and an active layer 108′ in which thelateral etching process is isotropic (same in all directions). Roughlyspeaking, if all vertical shafts are equally placed around a circle ofradius R2 centered on the current confinement zone 126′, and if and thelateral etching process forms lateral etches into the currentconfinement layer of average radius R1 (see the outline of a subsurfacecavity 206′ outwardly etched from the axis 204′), the currentconfinement zone 126′ will have an average radius of roughly R2-R1. Asindicated in FIGS. 2-3, the current confinement zones 126 and 126′ havea somewhat irregular geometric shape, but any sharp corners will likelybe dulled by the etching process.

FIGS. 4-7 show views of structures corresponding to different pointsduring a VCSEL fabrication process in accordance with a preferredembodiment. To fabricate a VCSEL in accordance with a preferredembodiment, the layers 102-116 are first generated using conventionalmethods through to the top DBR layer 116 (FIG. 4). FIG. 4 shows asimplified diagram of a vertical cavity structure prior to formation ofthe current confinement structure. Generally speaking, the currentconfinement layer 112 will be as close as possible to the active region108 of the VCSEL, so that the current does not appreciably spread outafter being confined and before passing through the active region 108.Thus, the layer beneath the current confinement layer will usually be anactive region layer, although the scope of the preferred embodiments isnot so limited. As shown in FIG. 5, the structure is ion-implanted toform the non-conductive implanted region 130, which is annularlydisposed around a center axis of the device.

As shown in FIG. 6, hollow vertical shafts 125 are then patterned on thesurface of the device beginning at the top DBR 116 and etched down untilthey reach at least the current confinement layer 112. As shown in FIG.7, a selective etchant is then used to etch only the current confinementlayer 112. The current confinement layer 112 is laterally etched awayoutwardly from the axis of each hollow vertical shaft 125, forming asubsurface cavity laterally around each hollow vertical shaft 125. Thelateral etching process is continued until the subsurface cavities mergetogether to form a single subsurface circumferential cavity 128 aroundthe desired current confinement zone 126.

Thus, the lateral etch proceeds until a desired amount of verticalcolumn material remains to form the current confinement zone 126. Theetches will usually be anisotropic along crystal directions of the layeretched, and therefore the etched regions are square or rectangular (orapproximately so) when viewed from the top (FIG. 2). At a minimum theetch should proceed until a convergence or merging is reached, in whichthe air cavity etched from each hole merges into the air cavity etchedfrom each neighboring hole to form the single subsurface circumferentialcavity 128. FIG. 7 shows a side view of the sub-surface structure afterthe etch process.

In some respects, the VCSEL fabrication process of the preferredembodiments is similar to that described in Choquette, supra, i.e. ahole defined oxide-aperture method, except that an etching step is usedinstead of an oxidation step. Advantageously, however, thecurrent-confining layer 112 can be made of any of a variety ofmaterials, whereas the current-confining layer in the oxide-aperturemethod must be readily oxidizable (e.g., AlGaAs). In the oxide-aperturemethod, the oxidation process requires a high Al content in AlGaAs, orAlAs, which have higher barriers to the current (electron/hole)injection process. This will result in a higher voltage for the deviceand hence a lower wall plug efficiency. According to a preferredembodiment, a lower-bandgap material than AlGaAs may be selected for thecurrent-confining layer 112, such as InGaAs (or related materials suchas InGaAsP). The use of this lower-bandgap material results in a lowervoltage across the current confining, region and therefore less heatingand higher wall plug efficiency. Because the current confinement zone126 is on the “p” side of the active region 108 in the embodiment ofFIG. 1, the current confinement layer should comprise a p-dopedsemiconductor material (e.g., p-InGaAs) or an undoped semiconductormaterial (e.g., i-InGaAs). The scope of the preferred embodiments is notso limited, however. In another preferred embodiment, the currentconfinement zone is on the “n” side of the active region, the currentconfinement layer comprising an n-doped semiconductor material or anundoped semiconductor material.

The material for the current-confining layer 112 should be selected suchthat it etches more quickly than the material above it in order for thesub-surface cavity structure to be properly formed, and to avoidundesired etching artifacts in the material above and below it. Becausethe respective layers are not “masked” along the vertical walls of theholes, it should be noted that the etching of current confinement layer112 may also result in some etching of the upper DBR mirror 116 and theactive layer 108. The current confinement layer 112 should be selectedsuch that the etch rate for this layer is significantly (at least afactor of 2 faster) faster than the etch rate of the surroundingmaterial. For example, an current confinement layer 112 composed ofAlGaAs with Al mole fraction of 0.92 or greater, will result in etchingof the current confinement layer 112 faster than the surroundingmaterial using buffered HF. InGaAs with an In mole fraction of 0.01 to0.2 may also be used as a current confinement layer 112. In cases wherethe active layer is composed of the same material as the currentconfinement layer 112, care must be taken in the positioning of thedepth of the vertical shafts such that it does not penetrate the activelayer. Etch stop layers can be used to prevent such penetration.

It is to be appreciated that the overall structure incorporating thesubsurface cavity is mechanically stable and resistant to collapse,since it is not a cantilever structure as in the case of the mesa methodof Deppe, supra. In addition, the overall wafer surface remainssubstantially planar which allows high-yield manufacturing.

As shown in FIG. 8, to passivate the sub surface air aperture, thevertical shafts 125 used to access the current confinement layer can besealed in a variety of manners, either gold plating to assist in heatremoval, dielectric seals using plasma enhanced chemical vapordeposition (PECVD), or polymer seal using polyimide. In addition, one ormore inert gases can be sealed inside the structure, such as N₂, Ar, orHe, for enhanced passivation or assistance in heat removal.

As described supra, the resulting cavity formed by the etching processwill be generally square or rectangular for an anisotropic etch, andgenerally circular or oblong for an isotropic etch. More generally,however, the air aperture may generally be circular, rectangular, or ofanother shape depending on the type of holes, the number of holes used,and the etching type and conditions. Optionally, the regions around thesub-surface air aperture may be filled with an inert gas which mayconsist of N₂, Ar and/or He for hermeticity and to help thermalconduction. Proton ion implantation is used also for isolation to reduceparasitic capacitances and to prevent undesirable current injectionpatterns from the etched trenches that may be filled with a conductingmaterial that may affect the mode pattern of the VCSEL.

In an optional preferred embodiment, multiple sub-surface air aperturesmay be used to achieve single spatial mode operation. Instead of etchingonly a single layer of material to form a single sub-surface cavity, aplurality of alternating layers of material may be etched to formseveral sub-surface cavities, in a manner that is perhaps analogous to amulti-level underground parking garage. Just as current confinementthrough multilevel oxidation may be advantageously used to suppresshigher-order modes in the vertical laser cavity (see Nishiyama, N. et.al., “Multi-oxide layer structure for single-mode operation in verticalcavity surface emitting lasers,” IEEE Photonics Technology Letters, Vol.12, No. 6, pp. 606-8 (2000), which is incorporated by reference herein),it has been found that multi-level subsurface cavity structures may alsobe used to suppress higher-order modes in the vertical laser cavity,thereby enhancing single-mode operation.

Many other VCSEL configurations are within the scope of the preferredembodiments, e.g., a top dielectric DBR and a top emitting VCSELstructure. A bottom emitting structure is shown for the device of FIG. 1because the active region is closer to the p-contact and heat can beremoved efficiently from the VCSEL by bonding the p-contact to a heatsink. However, it is to be appreciated that a person skilled in the artwould be readily able to adapt the methods and structures of thepreferred embodiments to a top-emitting structure.

By way of further example, in the device of FIG. 1 the active region is“below” the subsurface circumferential cavity. However, it is to beappreciated that a person skilled in the art would be readily able toadapt the methods and structures of the preferred embodiments to a VCSELin which the active region is “above” the subsurface circumferentialcavity. It may be advantageous to have the active region “above” thesubsurface circumferential cavity since then it is closer to thep-contact and the heat sink and has more material to help thermalconduction of the heat away from the junction toward the heat sink.

Whereas many alterations and modifications of the present invention willno doubt become apparent to a person of ordinary skill in the art afterhaving read the foregoing description, it is to be understood that theparticular embodiments shown and described by way of illustration are inno way intended to be considered limiting. By way of example, it is tobe appreciated that a person skilled in the art would be readily able toadapt the methods and structures of the preferred embodiments toshort-wavelength VCSELs, to VCSELs comprising any of a variety ofdifferent active region material systems, to VCSELs comprising any of avariety of different DBR materials, and in general to many differentkinds of VCSELs. Therefore, reference to the details of the preferredembodiments are not intended to limit their scope, which is limited onlyby the scope of the claims set forth below.

What is claimed is:
 1. A method of fabricating a current confinement structure into a partially-constructed VCSEL, said partially-constructed VCSEL having an active layer disposed between an upper set of material layers thereabove and a lower set of material layers therebelow, said partially-constructed VCSEL further comprising a current confinement layer positioned below the upper set of material layers and immediately above the active layer, comprising: forming at least three hollow vertical shafts extending downward from a top surface of the upper set of material layers to at least a bottom of the current confinement layer, the vertical shafts being laterally positioned outside a desired current confinement zone; and etching with a selective etchant that laterally etches the current confinement layer substantially faster than it etches any of the upper material layers and substantially faster than it etches the active layer, the current confinement layer thereby being etched away outwardly from an axis of each vertical shaft so as to form a subsurface void around each hollow vertical shaft at the current confinement layer, said etching continuing until the subsurface cavities merge together to form a single subsurface circumferential cavity around the desired current confinement zone.
 2. The method of claim 1, further comprising, prior to said forming the at least three vertical shafts, implanting said upper material layers and said current confinement layer in lateral areas lying outside the current confinement zone.
 3. The method of claim 2, further comprising subjecting the partially-constructed VCSEL to a predetermined surrounding environment while sealing off the hollow vertical shafts, said predetermined surrounding environment being sufficient to cause the subsurface circumferential cavity to contain a non-solid selected from the group consisting of: air, vacuum, and inert gas.
 4. The method of claim 3, wherein the hollow vertical shafts are sealed off with one or more sealing materials selected from the group consisting of: a polyimide material, a metallization material, and a dielectric material.
 5. The method of claim 1, wherein M>2 hollow vertical shafts are formed, the hollow vertical shafts being spaced in a roughly circular arrangement outside of the desired current confinement zone and being angularly separated by roughly 360/M degrees around a center of the roughly circular arrangement.
 6. The method of claim 5, wherein the number M of hollow vertical shafts is equal to
 4. 7. The method of claim 1, wherein the current confinement layer comprises an oxidation-resistant material.
 8. The method of claim 1, wherein the current confinement layer comprises a low-bandgap semiconductor material.
 9. The method of claim 1, wherein the current confinement layer comprises InGaAs. 